Atomic force microscopy of charge density waves and atoms on 1T–TaSe2, 1T–TaS2, 1T–TiSe2, and 2H–NbSe2

Abstract
In this paper, Atomic force microscopy (AFM) measurements on TaSe2 and 1T−TaS2 will be presented clearly resolving both the atoms with a z-deflection of ≊0.5 Å, and the charge-density waves (CDWs) also with a z deflection of ≊0.5 Å. All of the measurements have been carried out at room temperature using freshly cleaved crystals in air. It has been possible to distinguish very clearly between the commensurate CDW structure on 1T−TaSe2 and the incommensurate CDW structure on 1T−TaS2. For the commensurate CDW structure on 1T−TaSe2, the rotation angle between the atomic lattice and the CDW superlattice is measured as 14.2°±0.5. For the incommensurate CDW structure on 1T−TaS2, the rotation angle between the atomic lattice and the CDW superlattice is measured as 12.1°±0.3. A detailed analysis of the Fourier transformation suggest no appreciable regions of commensurate CDWs at an angle of 13.9°. Instead, the analysis favors a continuous modulation of the CDW amplitude, in agreement with recent STM data. The mechanism involved in the AFM response to the CDW structure and the magnitude of the z deflection will be discussed. In addition, high resolution AFM scans of atoms on 1T−TiSe2 and 2H−NbSe2 are presented that have no CDWs at room temperature.